Over the recent years a concept known as Free Space Optics has surfaced in the context of wireless broadband access as a method of providing high bandwidth communication connections to fixed locations, such as homes and offices. In prior art FSO schemes a laser beam between a transmitter and a receiver is used to connect a building to a communication network. A laser beam can carry information with a high bandwidth and can be used to provide IP and Ethernet communication, or any data communication to buildings.
Current FSO systems also have very short connections between transmitters and receivers, typically between 200-1000 m. This is primarily due to divergence and attenuation in prior art FSO links. Especially inclement weather such as fog, rain, mist or snow increases the attenuation, causes disturbances, and thus reduces the reliability of the FSO link. The prior art FSO links typically utilise one static laser at a fixed wavelength.
However, prior art also features a publication U.S. Pat. No. 5,966,229 where preferable wavelengths are deduced from an atmospheric spectral simulator, which may either simulate the atmosphere, or take a sample from 10-50 cm gas sample near the transmitter. The transmission wavelength is then tuned according to the measured spectrum. There is a serious shortcoming with this prior art method, the atmosphere is very inhomogeneous by nature, so no sample from a local optical path can really be representative of the spectrum of the path between the transmitter and the receiver, which is hundreds of meters or even kilometres. This document is cited here as reference. A method utilising the same concept appears also in publication WO 02/061959 A2, which is also cited here as reference.
A further problem with prior art FSO systems is that due to the short wavelength of the laser, line of sight is required between the transmitter and the receiver. This problem is especially highlighted in architectures where there is one central hub to several receivers. One inadequate attempt to solve this problem has been to arrange the transmitters and receivers in a mesh configuration, so that every receiver would see at least one other receiver, and thus have at least one active connection in the mesh. WO 00/25455 of Airfiber Inc is a good exhibit of some recently discovered concepts in FSO in accordance with the prior art, and is here cited as reference.
In cellular communication the line of sight problem is also encountered sometimes at high frequencies, and active repeater antennas are used to direct the radiation fields to areas where radio coverage is obstructed by an obstruction such as a building, rock etc. One alternative design for an active repeater antenna is exhibited in WO 01/17059 by Teligent Inc, which is cited here as reference.
The active repeaters have many shortcomings if applied to FSO links. The signal needs to be converted from optical to electrical and thus reduces the speed of the network. Prior art repeaters are also designed for a particular wavelength, i.e. they are dispersive. Power chords are also tedious to install to a network that has short connections, in the order of hundreds of meters.
The most obvious passive repeater at short wavelengths would of course be a mirror. However a mirror is difficult to design, difficult to focus, and mechanical drift very easily pushes the mirror from focus to out of focus, thus disrupting the link.
On the other hand telescopes have been known to transmit very parallel beams, by the process of beam expansion. For example, a telescope with an aperture of 1 m has been known to transmit a laser beam to the moon, with a diameter of 1 kilometer. (Optics and Photonics, F. Graham-Smith, T. A. King 2000). Further, U.S. Pat. No. 5,627,669: Optical transmitter-receiver, features a transmitter receiver where a beam expander has been integrated to provide a general small capacity optical communication within a short distance. Lucent Technologies has demonstrated telescopes in FSO systems in their WWW journal “Bell Labs Trends & Developments, Jun. 22, 2001.” Telescopes and beam expanders can be used to minimise divergence, but they do not reduce the attenuation of the beam.